1. Field of the Invention
The present invention relates generally to the field of graft polymerization and, more specifically, to graft polymerization of non-woven polyolefin fiber sheets, and to batteries in which graft polymerized non-woven polyolefin fiber sheets constitute separators which are positioned between the positive and negative plates.
2. Description of the Prior Art
Battery separators which are non-woven sheets of polyolefin fibers with an acrylic graft polymer on the fiber surfaces are known, being disclosed in a paper presented at the 1986 Power Sources Symposium held in Cherry Hill, N.J., J. Polym. Sci. 34, 671 (1959), and in WO 93/01622, published Jan. 21, 1993. The Symposium paper discloses the use of ionizing radiation to produce such separators, while WO 93/01622 discloses a method for producing such separators which involves immersion of non-woven sheets of polyolefin fibers in a solution of an acrylic monomer which also contains a photoinitiator, followed by ultraviolet irradiation of the sheets. Various publications, e.g., Yao and R{dot over (a)}nby, Journal of Applied Polymer Science, Vol 41, 1469-1478 (1990) disclose work carried out at the Royal Institute of Science, Stockholm, Sweden, which involved immersing polyolefin fibers and films in solutions of acrylic monomers and benzophenone, followed by ultraviolet irradiation of the fibers and films to produce an acrylic graft polymer on the surfaces of the fibers and films. Another publication, Journal of Polymer Science, Polymer Letters Edition, Vol. 19, pages 457-462 (1981) discloses the immersion of polyolefin films in an acetone solution of benzophenone and, after drying of the films, vapor phase and liquid phase copolymerization of an acrylic monomer with the polypropylene surfaces of the films to produce acrylic graft polymers.
A Journal article entitled xe2x80x9cChemical Absorptive Properties of Acrylic Acid Grafted Non-woven Battery Separatorsxe2x80x9d, refers to work carried out by Leblanc et al. (citing P. Leblanc, Ph. Blanchard, S. Senyarich, Abstract No. 261 ESC/ISE meeting, Paris, (1997) and P. Leblanc, Ph. Blanchard, S. Senyarich, Electrochem Soc,. 145, 846, (1998)) as showing that ammonia in a NiMH cell dramatically reduces the self-discharge performance, and cites the latter reference for the statements:
xe2x80x9cIt was also demonstrated that this effect could be significantly reduced if the free ammonia in the cell could be removed. The tests showed that by using an acrylic acid grafted separator with an ammonia absorption of over 1.5xc3x9710xe2x88x924 mol/g then all the free ammonia could be eliminated, and the self-discharge performance improved to the levels normally associated with NiCd cells.xe2x80x9d
The reference then refers to the process of WO 93/01622 as xe2x80x9ccapable of grafting non-wovens of all constructionsxe2x80x9d and states:
xe2x80x9cA study was carried out to examine the effect of the non-woven type on the separator""s ability to absorb ammonia using Kjeldhal""s technique (see tablet). All the grafted materials were grafted to the same level using the same grafting conditions. The results show firstly that an acrylic acid graft is necessary for a non-woven to posses the ability to absorb ammonia. Furthermore, the amount of absorption is a function of the base non-woven material. The strongest correlation is with the fibre size of the non-woven, and therefore also surface area, with fine separators absorbing the most ammonia.
xe2x80x9cSamples of the PP fine fibre spunbond were also prepared with two different graft levels, and their ammonia absorption measured (see table 2). These results show that the ammonia absorption is independent of the total amount of acrylic acid grafted onto the polymer backbone, and is further evidence that it is the base non woven which controls the degree of ammonia absorption.xe2x80x9d
The instant invention is based upon the discovery that a non-woven sheet of polyolefin fibers can be immersed in a solution of benzophenone, dried, immersed in a solution of acrylic acid, and subjected to ultraviolet irradiation, while the acrylic acid solution is still on its surfaces, to produce an acrylic graft copolymer on the surfaces of the fibers, and that the reaction proceeds more rapidly than when the non-woven sheet of polyolefin fibers is immersed in a solution of benzophenone and acrylic acid, and subjected to ultraviolet irradiation while the solution of acrylic acid and benzophenone is still on the surfaces of the fibers.
In a further aspect, the invention is based upon the discovery that a non-woven sheet of polyolefin fibers can be immersed in a solution of benzophenone, dried, immersed in a solution of acrylic acid, and subjected to ultraviolet irradiation, while the acrylic acid solution is still on its surfaces, and while the non-woven sheet with the acrylic acid solution on its surfaces is in contact with either air or an inert atmosphere, to produce an acrylic graft copolymer on the surfaces of the fibers.
In a still further aspect, the invention is based upon the discovery that a non-woven sheet of polyolefin fibers can be immersed in a solution of benzophenone, dried, immersed in a solution of acrylic acid, and subjected to ultraviolet irradiation, while the acrylic acid solution is still on its surfaces, and while the non-woven sheet with the acrylic acid solution on its surfaces is inside a polyethylene bag or tube, and is in contact either with air or with an inert atmosphere, to produce an acrylic graft copolymer on the surfaces of the fibers. In a preferred embodiment, after the volatile solvent in the photoinitiator solution vaporizes, a web of the polyolefin article is advanced through the solution of the acrylic monomer, and through a region where it is subjected to ultraviolet irradiation to cause the acrylic monomer to graft to the polyolefin surface, and sheets of a polyolefin such as polyethylene which are wider than the web are introduced above and below the web and the adjacent edges of the polyolefin sheets are sealed to one another to form a tube which surrounds the web, and it is the web surrounded by this tube which is advanced through the region where the web is subjected to ultraviolet irradiation.
In yet another aspect, the invention is based upon the discovery that the ammonia absorption capacity of a non-woven sheet composed of polyolefin fibers which have been graft polymerized with acrylic acid can be increased significantly by subjecting the sheet to corona discharge before the fibers are graft polymerized with acrylic acid.
In still another aspect, the invention is based upon the discovery of a method for producing a non-woven sheet of polyolefin fibers which is hydrophilic in at least one part and hydrophobic or hydrophilic to a different degree in at least one part. The sheet which is hydrophilic in at least one part and hydrophobic in at least one part can be produced by applying a solution of benzophenone or of another photoinitiator to a selected part or to selected parts of a non-woven sheet composed of polyolefin fibers, immersing the sheet, after evaporation of the solvent from the benzophenone or the like solution thereon, in an acrylic acid solution, removing the sheet from the acrylic acid solution, and irradiating the resulting sheet with ultra violet; the irradiation causes the acrylic acid to graft polymerize to the surfaces of the fibers where the benzophenone or the like solution was applied, making them hydrophilic, but does not cause graft polymerization to the surfaces of the fibers where benzophenone or the like was not applied, so that they remain hyrdrophobic. The sheet which is hydrophilic in at least one part and hydrophilic to a different degree in at least one part can be produced by applying a solution of benzophenone or of another photoinitiator to a selected part or to selected parts of a non-woven sheet composed of polyolefin fibers, applying a solution having a different concentration of benzophenone or of another photoinitiator to a different selected part or to different selected parts, immersing the sheet, after evaporation of the solvent from the benzophenone or the like solutions thereon, in an acrylic acid solution, removing the sheet from the acrylic acid solution, and irradiating the resulting sheet with ultra violet; the irradiation causes the acrylic acid to graft polymerize to the surfaces of the fibers where the benzophenone or the like solution was applied, making them hydrophilic, but the degree to which graft polymerization to the surfaces of the fibers makes them hydrophilic depends upon the concentration of the benzophenone or the like in the solution which was applied. When a recombinant battery is assembled in which non-woven sheets of polyolefin fibers which are hydrophobic in parts and hydrophilic in parts are used as a separator between adjacent plates, the hydrophilic portions of the sheets are wet by the electrolyte, but the hydrophobic portions are not. As a consequence, portions of the plates are wet by the electrolyte, which saturates adjacent portions of the separator, but gas evolved at one of the plates is free to migrate through the hydrophobic portions of the separator to an adjacent one of the plates.
Non-woven sheets of polyolefin fibers which can be used as starting materials in practicing the instant invention are produced commercially by numerous processes which have been classified as xe2x80x9cdry-laid systemsxe2x80x9d and as xe2x80x9cmelt spun systemsxe2x80x9d. Recognized xe2x80x9cdry-laid systems are known as xe2x80x9crandom air laidxe2x80x9d, as xe2x80x9ccarded web systemsxe2x80x9d and as xe2x80x9cspun lace systemsxe2x80x9d, while recognized xe2x80x9cmelt spun systemsxe2x80x9d are known as xe2x80x9cspunbondedxe2x80x9d and xe2x80x9cmelt blown processes. These methods are all well known, and are disclosed in the literature, e.g., in xe2x80x9cIntroduction to Nonwovensxe2x80x9d, Albin Turbak, TAPPI Press, Atlanta, Ga., 1998 (see, in particular, pages 32-39).
It is, therefore, an object of the invention to provide an improved method for producing a graft polymer on the surfaces of polyolefin fibers and films.
It is another object to provide an improved method for producing an acrylic graft polymer on the surfaces of polyolefin fibers and films.
It is a further object to provide a method for producing a non-woven sheet of polyolefin fibers which has at least one region in which the fibers are hydrophilic as a consequence of an acrylic acid graft polymer on fiber surfaces and at least one region in which the fibers are hydrophobic.
It is still a further object to provide a non-woven sheet of polyolefin fibers which has at least one region in which the fibers are hydrophilic as a consequence of an acrylic acid graft polymer on fiber surfaces and at least one region in which the fibers are hydrophobic.
Other objects and advantages will be apparent from the description which follows, which is intended only to illustrate and disclose, and not to limit, the invention.
In the following Examples, and elsewhere herein, the terms xe2x80x9cpercentxe2x80x9d and xe2x80x9cpartsxe2x80x9d refer to percent and parts by weight, unless otherwise indicated, and xe2x80x9cgxe2x80x9d means gram or grams, xe2x80x9cmlxe2x80x9d mean milliliter or milliliters, xe2x80x9cmxe2x80x9d means meter or meters, xe2x80x9ccmxe2x80x9d means centimeter or centimeters, xe2x80x9cmmxe2x80x9d means millimeter or millimeters, xe2x80x9cxcexcmxe2x80x9d means micrometer or micrometers, xe2x80x9cKVxe2x80x9d means kilovolts, xe2x80x9cKWxe2x80x9d means kilowatts.